1. Field of the Invention
The invention relates to superheterodyne receivers and in particular relates to superheterodyne receivers which can receive signals on either of two separate frequencies.
2. Description of the Prior Art
Conventionally radio receivers which received signals on two separate frequencies have done so in a sequential fashion. Either the receiver could be manually tuned from one frequency to another or the receiver automatically and periodically sampled each of two or more frequencies. To build a receiver which is capable of receiving either one of two FM signals in a VHF band typically required one crystal for each frequency to which the receiver could be tuned. Highly expensive circuitry could eliminate the need for individual crystals but the resultant cost was increased. For radio receivers designed to monitor weather bureau frequencies of 162.4 MHz and 162.55 MHz, receivers required two separate crystals. While single frequency receivers could be designed to receive signals on either of the two weather frequencies, these single frequency receivers presented problems in marketing since it depended upon the location of the user as to which of the two frequencies was more appropriate.
Radio receiving circuits are known which do receive signals on two separate frequencies. Such radios are normally referred to as diversity radios. FIG. 4 of U.S. Pat. No. 2,955,199 discloses a circuit for receiving a signal on either one of two frequencies. In this circuit an oscillator located precisely midway between the two frequencies produces an IF signal. The amplitude modulation of the IF signal is detected and used to control the phase of the local oscillator. This then assures that the local oscillator functions precisely between the two received signals. This receiver is intended to be used in a diversity system in which two identically modulated signals are normally present on the two received frequencies. The precise phase control allows additive combination of the two signals without substantial distortion. It is therefore essential that the local oscillator be controlled by the incoming signals. A lack of control would defeat the improvement of the signal-to-noise ratio which is one of the advantages of a diversity system.